During the movie, Michael Tight is tied up on a leather chair by 'thugs' and forced to have sex with the blonde star, who has starred in more than 100 porno movies.

Filmakers paid more than £1,000 to hire the venue and the film was shown this week on the pay-per-view Adult Channel - attracting an audience of several thousand people.

But trustees at Fort Amherst in Chatham, Kent - Britain's best surviving Napoleonic fortress - are furious that the filming was allowed to go ahead.

The film was shot in just 10 hours amongst the many tunnels at the historic fort - which only re-opened fully in May this year after a £50,000 grant from the Heritage Lottery Fund.

The fort is also part of Medway Council's bid to get the area listed as a World Heritage Site.

Speaking this week, trustees of the Fort Amehurst Heritage Trust - a charitable trust - said the filming should never have taken place - and it is the chairman of the Trust at the time, Martin Rogers, has now left his post.

Trustee Matthew Hill said this week: "I think it absolutely appalling - there must be better ways of raising funds through filming rights."

He said he was demanding to know why filming was allowed and said he was urging the new chairman Edmund Gulvin to conduct a full investigation.

He told the Medway Messenger newspaper: "I think if you want to film a porn film do it in a warehouse or wherever they do it."

One volunteer at the fort, who did not want to be named, said this week: "It is disgusting that the fort has been used for the backdrop of a pornographic movie.

"How it ever happened it a mystery to me, but it certainly made me feel a bit seedy when I went back to work."

Trust chairman Edmund Gulvin said: "Fort Amherst confirms that a professional film company in the adult entertainment industry filmed scenes at Fort Amherst during May this year.

"The contract was handled by a third party location company employed by us and was a private hiring of the fort.

"The filming was conducted under a closed set policy and there was no public access to the set at any time during the filming."

The fort was built in 1755 to protect Chatham Dockyard from a Napoleonic invasion, but is now a tourist attraction described by English Heritage as 'the most complete Napoleonic fortification in Britain'.

It is regularly used by paranormal investigators, who hire it for between £350-£500 a night to conduct investigations and it is also widely used by battle re-enactment societies.

The fort - which has 14 acres of batteries, bastions and tunnels -.was also used in the in the 1986 film The Mission starring Robert De Niro and Jeremy Irons.